A plate for a lead-acid battery is conventionally made by applying a battery paste to a conductive support such as a lead alloy grid. The paste is made from lead oxide, sulfuric acid and water. The lead oxide reacts with the sulfuric acid to form mono-, tri- or tetrabasic lead sulfate(s). Dry additives such as fiber and expander may be added. The mixture is then dried and water is re-added to form a paste of the desired consistency. The paste is applied to the lead grid, and the pasted plates are then flash-dried and cured at an elevated temperature and humidity to oxidize free lead and adjust the crystal structure of the plate. After curing, the positive plates are assembled into batteries and electrochemically formed by passage of current to convert the lead sulfate or basic lead sulfate(s) to lead dioxide, thereby forming the active lead material.
The lead alloy grid is made in a multistage process in which molten lead alloy is cast to form a strip. The strip is expanded to form mesh elements for grids, and the paste is then applied. Liner papers are applied to the surfaces of freshly pasted plates during the pasting operation to facilitate handling and stacking. Individual plates are formed by passing the pasted strip through a rotary divider that cuts the outlines of the individual plates on the moving strip. To ensure proper registration for cutting, the strip is formed during expansion with a series of regularly spaced central recesses that are engaged by lugs of a drive roller associated with the divider. The cut plates are then flattened, flash-dried, and stacked for later use in lead-acid battery manufacture.
The battery grids each take the form of a generally rectangular frame supporting a mesh of grid elements onto which the paste is applied. The frame has a pair of upper corners and a pair of lower corners, and an electrically conductive tab extending from a location between the two upper corners. In one common battery design, every other plate in the battery stack is inserted into an envelope made of a separator material such as submicro polyethylene. The sides of the envelope act as separators between the plate in the envelope and the two adjoining plates in the battery stack.
In assembling a battery of this kind, it is necessary to insert the battery plate bottom-first into the open end of the envelope so that the conductive tab at the top of the plate extends out of the envelope. However, the bottom corners on the battery plates are sharp, and will snag and tear the separator material between the positive and negative plates, causing an electrical short within the battery and reducing battery life. Bending or vibration of the plate disposed in the envelope during assembly or use can also cause tearing, and the problem is not confined to envelope-style separators.
Use of battery plates with rounded bottom corners would eliminate the snag and tearing of separators, but no practical process has been proposed for producing rounded corners on such battery plates. In particular, any process wherein the rotary divider cuts off corners results in small pieces of trim (scrap) that are severed from the strip. These pieces, if left on the strip after cutting, can come loose and cause cutting die failure on the divider and can lead to battery failure if the pieces are carried over and inadvertently incorporated into the finished battery. The present invention addresses these problems by providing a system for cutting the rounded corners and then removing the pieces during continuous plate manufacture.